feat(xlang): refine xlang api

[chaokunyang]

Why?

• Java enums needed a stable numeric tag option for cross-language and schema-evolution scenarios that should not depend on declaration order.
• The xlang implementation guide and related docs/specs had drifted from the current runtime architecture and API surface.

What does this PR do?

• Remove ForyBuilder.Xlang(bool) from the C# public API, make Config builder-created only, and make C# always emit and require the xlang frame header.
• Add Java @ForyEnumId support so enums can use a single annotated field, a zero-argument getter, or per-constant annotations for stable numeric tags, with dense-array lookup for small IDs and sparse-map lookup for larger IDs.
• Add Java serializer and xlang tests covering default ordinal behavior, explicit enum IDs, validation failures, and cross-language enum evolution with stable IDs.
• Refresh the C# guides and Java/xlang specs, including a rewrite of the xlang implementation guide around the current Fory / WriteContext / ReadContext / RefWriter / RefReader architecture.

Testing

• cd csharp && dotnet format Fory.sln --verify-no-changes
• cd csharp && dotnet test Fory.sln -c Release --no-restore
• cd java && export JAVA_HOME=/Library/Java/JavaVirtualMachines/graalvm-25.jdk/Contents/Home && mvn -T16 -pl fory-core spotless:check
• cd java && export JAVA_HOME=/Library/Java/JavaVirtualMachines/graalvm-25.jdk/Contents/Home && mvn -T16 -pl fory-core test -Dtest=org.apache.fory.serializer.EnumSerializerTest,org.apache.fory.xlang.EnumTest
• cd python && python -m pip install -v -e .
• cd java/fory-core && export JAVA_HOME=/Library/Java/JavaVirtualMachines/graalvm-25.jdk/Contents/Home && FORY_CSHARP_JAVA_CI=1 FORY_PYTHON_JAVA_CI=1 ENABLE_FORY_DEBUG_OUTPUT=1 mvn -T16 test -Dtest=org.apache.fory.xlang.CPPXlangTest,org.apache.fory.xlang.CSharpXlangTest,org.apache.fory.xlang.RustXlangTest,org.apache.fory.xlang.GoXlangTest,org.apache.fory.xlang.PythonXlangTest

Related issues

AI Contribution Checklist

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Does this PR introduce any user-facing change?

• Does this PR introduce any public API change?
• Does this PR introduce any binary protocol compatibility change?

Benchmark

N/A